#include <bits/stdc++.h>
using namespace std;
ifstream fin("sate.in");
ofstream fout("sate.out");
vector <pair<int, int> > h[30001];
queue<int> Q;
int d[30001],X,Y; /// d[i] = distanta de la x la i
int main()
{
    int i,j,dist,m,n,x,y;
    fin>>n>>m>>X>>Y;
    for(i=1; i<=m; i++)
    {
        fin>>x>>y>>dist;
        /// pun in h[x] perechea y dist
        h[x].push_back({y,dist});
        h[y].push_back({x,-dist});
    }
    for(i=1; i<=n; i++)
        d[i]=-1;
    d[X]=0;
    Q.push(X);
    while(!Q.empty())
    {
        x=Q.front();
        Q.pop();
        for(auto w : h[x])
        {
            j=w.first;
            dist=w.second;
            if(d[j]==-1)
            {
                d[j]=d[x]+dist ;
                Q.push(j);
            }
        }
        if(d[Y]>=0)
        {
            fout<<d[Y];
            return 0;
        }
    }
    return 0;
}